Implement the measure operator

[dominiklohmann]

The measure [--real-time] operator emits metrics about the input into the pipeline. The operator handles both events and bytes as input, and emits the metrics as events as described by these schemas, respectively:

type vast.metrics.events = record  {
  timestamp: time,
  schema: string,
  schema_id: string,
  events: uint64,
}

type vast.metrics.bytes = record  {
  timestamp: time,
  bytes: uint64,
}

The --real-time option causes the operator to instantly emit the metrics at the cost of throughput.

### Tasks
- [x] Implement the operator
- [x] Agree on schema names for the metrics events
- [x] Write tests
- [x] Document the operator
- [x] Add a changelog entry

[jachris]

One general comment: You could consider renaming this operator, because inspect is not the most intuitive name for its behavior. Perhaps even something like metrics (although we'd normally avoid nouns). But you can also keep it as-is.

[dominiklohmann]

@jachris: You could consider renaming this operator, because inspect is not the most intuitive name for its behavior. Perhaps even something like metrics (although we'd normally avoid nouns). But you can also keep it as-is.

@mavam do you have an idea for a better name? I agree generally that inspect is not the ideal name, but I don't know what we should use. Potential candidates that are verbs:

1. inspect
2. measure
3. debug
4. monitor
5. view (similar to the UNIX tool pv)
6. metric (which according to Wiktionary can also be used as a verb)

[jachris]

TBH, although I absolutely prefer verbs as well, we should not put this requirement above everything else. If there is no adequate verb, we should just use a noun. And we already do this with from and to (although you might argue that sources and sinks are special). Also, we might want to use inspect for something else that is more inspect-y.

[mavam]

I agree that inspect is probably more well-suited to yield the AST of the pipeline or some other structural information. Ideas for verbs, in order of preference:

1. profile
2. measure
3. observe
4. monitor

[dominiklohmann]

Of the suggested alternatives I chose to go with measure for now; there was no objection internally.